1. Field of the Invention
This invention relates to a constant current source circuit, and more particularly, to a constant current source circuit suitable for a bias current supply source of a monolithic integrated circuit (IC).
2. Description of the Prior Art
Constant current source circuits are very useful in integrated circuit (IC) design. Many forms of constant current source circuits have been developed. In constant current source circuits it is required that the operating current of each circuit not be subjected to changes such as undesired current variations (referred as noise signals hereafter), for example, a variation in the power source voltage. Constant current source circuits are also necessary so that the IC can operate at a low power supply voltage and exhibit good power consumption characteristics.
A typical example of a prior art current source circuit is illustrated in a circuit diagram of FIG. 1, which is similar to a circuit shown in "Analysis and Design of Analog Integrated Circuits" by Paul R. Gray and Rober G. Meyer, published by John Wiley & Sons (1977), pp. 200 to 206 and pp. 236 to 241.
As shown, an unstabilized current source 11 has one end connected to a power supply terminal 12 with a voltage Vcc, and the other end connected to the collector of a first transistor 13. The first transistor 13 has an emitter connected to a common power supply end 14 with a ground potential, and a base short-connected to its collector. A second transistor 15 has its base connected to the base of the first transistor 13, its emitter connected to the common power supply end 14 via a resistor 16, and its collector connected to an output terminal 17. A load (not shown) may be connected between the power supply terminal 12 and the output terminal 17, and an output current Iout serving as a load current is then supplied to the load.
The operation of this prior art circuit will now be described. In the circuit of this kind, the collector potential of the first transistor 13 is regulated by the base to emitter voltage Vbe13 of the first transistor 13, since the base of the first transistor 13 is connected to its collector, as described before. Therefore, the first transistor 13 provides a stabilized collector voltage. The base to emitter voltage Vbe13 on the collector of the first transistor 13 is applied to the base of the second transistor 15. On the other hand, the base potential of the second transistor 15 is given a sum of the base to emitter voltage Vbe15 of the second transistor 15 and the voltage drop V16 across the resistor 16. Therefore, a following equation is obtained. EQU V16=Vbe13-Vbe15=.DELTA.Vbe (1)
Here, the difference voltage .DELTA.Vbe in the Equation (1) is given as follows. ##EQU1## where k is Boltzman's constant;
T is an absolute temperature; PA1 q is the electron charge; PA1 VT (VT=kT/q) is the thermal voltage; PA1 I11 is an input current supplied to the constant current source by the current source 11; and PA1 N is an emitter area ratio of the second transistor 15 to the first transistor 13.
If the transistors Q13 and Q15 have large current amplification factors .beta.13 and .beta.15, base currents of the transistors Q13 and Q15 can be neglected (this assumption is valid for primary approximation since NPN transistors generally have a current amplification factor .beta. of 100 or more). In this condition, the output current Iout can be expressed as follows, ##EQU2## where R16 is a resistance of the resistor 16.
If the input current I11 is not sufficiently stabilized so that a variation component or a noise current I(n)11 is included in the input current I11 supplied by the current source 11, the constant current source circuit outputs an output noise current I(n) out as follows. EQU I(n)out=I(n)11/[1+logn(I11/Iout).multidot.N] (4)
As is clear from the Equation (4), the output noise current I(n)out can be reduced by increasing the emitter area ratio N. For example, if the emitter area ratio N is set to a value of 148 (i.e., N=148) and it is intended to obtain an output current Iout equal to the input current I11 supplied by the current source 11, the output noise current I(n)out is given as follows. ##EQU3## That is, the output noise current I(n)out can be reduced to 1/6 of the input noise current I(n)11.
Referring now to FIG. 2, a graph of the reduction rate RN of the output noise current I(n)out to the input noise current I(n)11 is shown as a function of the emitter area ratio N. As seen from FIG. 2, the effect of the noise reduction increases exponentially inproportion to the emitter area ratio N.
In the prior art constant current source, however, it is difficult to increase the emitter area ratio N to a desired sufficiently large ratio, such as the ratio 148. This is because that the second transistor 15 would become too large for practical use. In particular, it is impossible to fabricate a transistor having such a large emitter area on an integrated circuit chip.